A point defect model for YBa2Cu3O7 from density functional theory

The advent of high temperatures superconductors (HTS) will enable the development of compact fusion reactors capable of delivering large quantities of low carbon energy. However, the inside of a fusion reactor is a hostile environment and bombardment by high energy neutrons will alter the microstructure of constituent materials, including the HTS in the magnets. Key to understanding the evolution of a material's microstructure when subjected to neutron irradiation is knowledge of the defect population in the starting material. Therefore, this work uses density functional theory (DFT) simulations, combined with simple thermodynamics, to create a point defect model that enables prediction of the types and concentrations of defects present in a model HTS, YBa2Cu3O7, under a range of fabrication conditions. The simulations predict that the defect chemistry of YBa2Cu3O7 is dominated by oxygen defects, predominantly vacancies, in agreement with prior experimental observations. Interestingly, the simulations predict that the exchange of Y and Ba atoms is the second lowest energy defect process in YBa2Cu3O7 after the oxygen Frenkel process. Furthermore, the point defect model shows that any cation non-stoichiometry will also be accommodated via antiste defects rather than either vacancies or interstitials. Overall, these results suggest that future fusion magnets will contain a high concentration of oxygen and cation antisite defects and that these must be considered in future studies of the evolution of HTS materials under irradiation.